Does putting a signal from an audio interface through a mixer reduce sound quality ?

Hello, sorry in advance for the longer post.

I recently bought an Allen & Heath ZED-12FX mixer because I needed a mixer with three stereo channels. I use my room for recording and enjoying music. So one channel is meant for my audio interface (UA Apollo Twin USB), one for my CD player (Yamaha CDX-596), and one for my turntable (Audio Technica LP-140XP). After plugging in the devices, I send the main output to my monitors (KRK Rokit RP7s with a KRK subwoofer). I did this so I can switch between my interface, turntable and cd player without having to plug the cables in and out everytime from my monitoring/sound system.

After connecting everything, I listened to each individual stereo channel. The vinyl on the turntable sounds great, and the CD player sounds good; I just needed to boost low frequencies a tiny bit. However, I hear a significant difference between the sound of my interface plugged in directly to the monitors and the signal going through the mixer to the monitors. The sound is quiet and lacking in low end. I noticed that there is some distortion added due to increasing gain on the mixer. So I adjusted the levels by turning up the signal from the interface and the monitors while turning down the gain and fader on the mixer. This improved the sound quite a bit, but there was still the problem with the low end. I increased the low end on the mixer, and now it sounds fine. I am 90% back to the sound I had while connecting the interface directly to the monitors. The last 10% is clarity; when I turn the signal up loud, it lacks the clarity it had with directly plugging it into the monitors.

So I am asking you, is there some crucial step I missed? If not, do you have any tips on how to get a better sound?